Grammar - passive voice

study ex 1 ex 2 ex 3 ex 4
 
1. Form
We form the passive like this: to be + past participle.
Present simple: Diamonds are found in South Africa.      
Past simple: The goods were delivered yesterday.      
         
2. Use
We use the passive when it is not important who does the action,
or when we don't know who it is:
e.g. This castle was built in the 16th century (we don't know who built it).

We also use the passive because we have been talking about something,
and not the person who did it. We use 'by' to say who does/did the action.
e.g. This film was made in 1956. It was directed by Hitchcock.
 
3. Active and passive
Passive verbs have the same tenses as active verbs:
  Active Passive  
Present simple She cleans the room every day. The room is cleaned every day.  
Past simple He cleaned the room yesterday. The room was cleaned yesterday.